Piezoelectric devices, such as piezoelectric quartz filters, piezoelectric quartz resonators and the like, typically include a piece of piezoelectric material mounted to a substrate. In quartz devices, the quartz element of necessity has thin metallic electrodes attached to it through which electrical signals are coupled into and out of the piezoelectric quartz material. Common problems with piezoelectric devices are adequately isolating the piezoelectric device from mechanical shock and dealing with thermal expansion coefficient mismatches between the piezoelectric material and the substrate material.
Quite often, the piezoelectric devices, such as a piezoelectric quartz material and the substrate have mismatched thermal expansion coefficients. This mismatch can cause mechanical stresses to be induced in the quartz as time goes by during the life of such devices, as the quartz and substrate expand and contract over temperature variations. Further, mechanical shock transferred to the quartz through its mounting structure can increase mechanical stresses that in addition to the thermal stress, adversely affect the frequency, accuracy and longevity of such devices.
Various attempts over the years have been developed to compliantly mount piezoelectric quartz devices to a substrate. For purposes of this application, a compliant mount for a piezoelectric device, is a mounting device, apparatus or other mounting means that attempts to reduce or minimize mechanical stresses on the piezoelectric quartz element. Some prior art compliant mounting devices have used thin foil tabs that act as spring-type mounting structures that attempt to isolate the quartz element from its substrate. Other types of compliant mounting structures have attempted to use substrate materials having thermal expansion coefficients which more closely match the thermal expansion coefficient of the quartz material itself.
Examples of two prior art devices are shown in FIG. 1a and FIG. 1b.
In FIG. 1a, a loose tolerance clock oscillator 10 is shown. It is made as follows. Epoxy 12 is deposited onto a ceramic package 14. A quartz crystal 16 is placed onto the epoxy 12 and the epoxy 12 is cured in an oven. Two time consuming wirebonds 18 are then made between the quartz 16 and the ceramic 14. The coupling between the quartz 16 and the package 14 is rigid and direct and the frequency stability of the device is marginal, requires meticulous detail to assemble, and is not easily adapted to mass production. Also mechanical shock performance is poor because the mounting is rigid and the wirebonds can easily break.
In FIG. 1b, a cantilever type crystal mount 20 is shown. It is made as follows. First, epoxy 22 is dispensed on a left side of the ceramic 24. Next, a crystal 26 is suitably placed so that a bottom electrode 30 contacts epoxy 22. Thereafter, the cantilever mount 20 is turned upside down and inserted into a mating tool that sets the gap between the crystal and the ceramic 24 at the other side 28 at about 0.003" (0.08 mm). The cantilever mount 20 is then cured in an oven while upside down in the mating tool. Thereafter, an upper electrode 32 of the crystal 26 is coupled to a lead in the left side of the ceramic 24 with a conductive adhesive 34 extending from the upper electrode 34 to the lead (not shown) on the left side of the ceramic 24. The adhesive 34 is then cured. This operation is very difficult if not impossible to automate, and the cantilever mount 20 has marginal mechanical shock performance.
Most if not all of the prior art compliant mounting schemes are difficult to use because of the small physical dimensions that modern piezoelectric quartz elements have. Using bent foil tabs alone, for example, to compliantly mount a small sliver of quartz onto a substrate is not a structure that lends itself to economic mass production of quartz crystal devices.
There is a need for an improved method of mounting piezoelectric elements with ceramic substrates: (i) to minimize the mechanical stresses induced due to the thermal expansion mismatch between the two; (ii) to provide a mechanically sufficient coupling such that the device can withstand mechanical shocks; and (iii) to provide a method of crystal attachment which is adaptable to mass production. Accordingly, a low cost, readily-manufacturable, compliant mount for a piezoelectric device would be an improvement over the prior art. A method by which quartz devices can be easily and reliably attached to a substrate and which isolates the quartz element from mechanical stresses would be an improvement over the art.